Chair base



FIG. 1 is a perspective view showing the top, front and left and right sides of a chair base which forms my new design;

FIG. 2 is a bottom view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a front elevational view thereof;

FIG. 5 is a rear elevational view thereof;

FIG. 6 is a left side elevational view thereof; and,

FIG. 7 is a right side elevational view thereof. 

The ornamental design for a chair base, as shown and described. 